In the recovery of oil from oil-bearing reservoirs, it is often possible to recover only a portion of the oil contained in the underground formation by the so-called primary recovery methods which utilize the natural forces present in the reservoir. Exceptionally poor oil recovery can be due to a number of factors including the breakthrough of an overlying gas body or an edge or bottom water to the production well by coning or channeling. This is particularly evident where reservoir heterogeneities such as fractures or high permeability streaks are selectively depleted of oil allowing the premature entry of adjacent gas or water into the production zone. In heavy oil fields, the relatively low viscosity and high mobility of a water phase allows such breakthrough even in homogeneous reservoirs by channeling or fingering of the water through the relatively immobile oil phase. Once highly water permeable channels are established, heavy oil production is lost.
A variety of enhanced recovery techniques (e.g., secondary and tertiary recovery) have been employed in order to increase the recovery of oil from subterranean reservoirs. In one form of enhanced recovery of oil, a drive fluid is injected under pressure into the oil reservoir through one or more injection wells to maintain, restore or produce formation pressure. The most widely used drive fluid is water, however, more complex aqueous systems, solvents and gases are also useful as drive fluids. Steam is often employed for heavy oils. The drive fluid is frequently introduced into the oil-bearing underground formation near the bottom of the formation at or above formation pressure, to displace oil in the reservoir. As the fluid moves through the reservoir, it drives or flushes the oil through the formation. An increased oil saturation develops ahead of the moving fluid and finally reaches the production well or wells.
Generally, an oil-bearing underground formation will consist of various regions having different permeabilities. Drive fluid moving through the reservoir preferentially moves to and through regions of higher permeability, fractures and the like. Drive fluids will pass predominantly through such channels bypassing regions of lower permeability and, thus, bypass oil contained in such lower permeability regions, thus reducing the sweep efficiency of the displacing medium.
Prior art techniques for homogenizing permeability, and thus increasing sweep efficiency, often require the injection of noxious chemicals and/or polymers.
Additionally, the problems associated with enhanced oil recovery can be particularly acute when the oil is the highly viscous, so-called “heavy oil” which exists in the Lloydminster region in Canada and in certain reservoirs located in Alaska and Venezuela. In such heavy oil reservoirs, primary recovery and conventional water flooding operations are sometimes estimated to produce as little as 4% to 8% of the oil contained in the reservoir.
Moreover, while U.S. Pat. Nos. 4,460,043; 4,561,500; and 5,143,155 disclose that oil production may be enhanced through the application of a selective microbial plugging system, these microbial plugs are susceptible to damage caused by high velocity fluid flows or large pressure drops across the plug, by thermal degradation in steam drive situations and the like, and by the degradation of biological plugging materials by the indigenous microbial population. Furthermore, such plugs may not be easily removed. Once the oil in the low permeability channel is expressed, this results in increased backpressure at the injection well-head that increases the risk of permanent plugging of the local environment around injection well and unpredictable fracturing of the reservoir stratum.
Accordingly, there exists a need for improved methods of enhanced oil recovery that homogenize the reservoir permeability, which result in a significant improvement in oil recovery regardless of oil viscosity and/or characteristics (i.e., both light and heavy oil); that are not susceptible to thermal damage or break down as a result of large pressure drops across the plug; that are not susceptible to either biological or chemical degradation by the indigenous microbial population; and that reversible so as to allow for the sequential expansion of the permeability homogenizing effect out into the reservoir matrix to maintain increased sweep efficiency without risk to the reservoir integrity.